Heat retentive servers are commonly used in hotels, institutional environments such as hospitals and nursing homes, and like operations to keep food warm prior to serving. Frequently there are substantial delays between the time the food is removed from the oven and the time it is actually served. Such delays may, for example, commonly exceed thirty minutes by which time the food is cold. Accordingly, various devices for keeping food warm until it can be served have been commercially available and have been suggested in prior art literature. Heat retentive servers generally include a server base and a dome for such base. One or both of the base and dome is typically insulated so that food held between the base and dome will stay warm for a desired time period. When the server base is designed to support dishware, which in turn holds food, such a base is referred to as a pellet base and the entire system, i.e., base, dome and plate, is referred to as a pellet system. Prior art server bases and domes have also been designed to include heat retention mediums such as solid heat sinks. When a heat sink is incorporated into a server base and the base supports a food carrying plate, the base can be referred to as a plate warmer.
Prior art plate warming devices, which include a heat storage server base or dome having a heat sink disposed between the upper and lower walls of the base or dome, have taken into account special considerations. More specifically, in use, the heat storage base or dome is initially heated to store heat in the heat sink, and thereafter, when a plate of food is placed on the heat storage base or under the heat storage cover, the plate and food are kept warm by the heat passively released from the heat storage sink. In such devices, the sink is generally formed of a solid metal. The size of the sink is thus limited and generally occupies only a portion of the interior space of the base or dome. The air trapped in the remaining space between the base or dome walls expands when the base or dome is heated, so that means must also be provided to relieve the internal pressure in the base or dome resulting from air expansion and thereby prevent the base or dome from bursting.
One prior art structure directed to this air expansion problem is disclosed in U.S. Pat. No. 3,557,774. In the '774 patent, the bottom wall of the heat storage server base includes an elevated annular wall portion which is deformable as the air in the space between the walls expands. However, one disadvantage of the base is that it requires a complex bottom wall, which requires complicated fabrication and assembly, and therefore is not particularly suited for mass production.
Another attempt to resolve the air expansion problem may be found in U.S. Pat. No. 4,086,907 which includes indents or corrugations in a concave bottom base wall to permit expansion or deformation of the bottom wall, and thus, prevent the base from bursting should the base be overheated. However, the device not only suffers from the disadvantage of requiring relatively complicated fabrication and assembly, the construction itself presents certain problems during use. For example, the concave configuration of the base portion provides substantial resistance to expansion under normal conditions. The spot welds which secure channel members extending through slots in a metal heat sink to the bottom side of the top wall are susceptible to breakage due to heat stress over continued recycling.
Another disadvantage of servers which use metal heat sinks is that because of the relatively high thermal conductivity of metals such as aluminum, the heat storage base or cover, when heated to a relatively low temperature, for example, 230.degree. F., is limited with respect to the amount of time it is effective to keep food warm. Although the heat storing server may be initially heated to a relatively high temperature, i.e., in excess of 350.degree. F., to store sufficient energy in the metal heat sink to keep food warm for an appreciable period of time, this, of course, increases the inherent risk in handling such servers and increases the risk that the server may burst. Also, while the heat sink can be increased in size to store more heat, the physical size and weight limitations for devices of this type generally do not permit increasing the size of the heat sink.
U.S. Pat. No. 3,148,676 discloses a food warming device wherein the metal heat sink is replaced by a phase change material such as a wax or asphalt substance having a relatively high specific heat and a relatively low melting point, e.g., between 180.degree. and 270.degree. F. The substance may be a wax such as carnauba wax, Cornox wax or a synthetic hardened microcrystalline wax, and stores a relatively large amount of heat energy which is gradually released at a rate which is much less than the rate at which it was stored. The substance fills a chamber between the top and bottom walls of the unit and is retained within a honeycomb framework which is fabricated from aluminum or the like to form a multiplicity of relatively small, closely spaced cavities in the chamber. Expansion of the substance is accommodated by a pair of spaced circular lines of weakness in the annular recessed portion of the top wall which provide relief means for preventing the unit from bursting in the event that excess pressure is developed in the chamber. This unit, thus, also requires relatively complicated fabrication and assembly.
Fabrication of units utilizing heat storing substances, such as those disclosed in U.S. Pat. No. 3,148,676, is also difficult because the heat storing substance is not readily insertable into the unit in its solidified state where, for example, a honeycomb framework or the like is required. The honeycomb framework has an open top and bottom. If the substance is first melted for insertion into the honeycomb framework, the substance must be allowed to cool within the partially assembled server before further fabrication or assembly can be undertaken. Since one advantage of the heat storing substance is its capacity for heat retention for long periods of time, it is some time before the substance has cooled sufficiently to permit further work. Moreover, if the melted substance is injected into the device, the injection hole must be sealed, such as by soldering, and if the hole is improperly sealed, the seal may rupture due to expansion of the heat storing substance during use, allowing the substance to leak from the device or allowing water or air to migrate into the chamber. In either case, mass production of such units is restricted.
Further prior art efforts to solve the aforementioned problems still suffer from other disadvantages. For example, U.S. Pat. No. 4,246,884 discloses a plate warmer including a stainless steel outer shell having an inwardly concave top wall and an opposing inwardly concave bottom wall joined thereto by an interconnecting peripheral side wall. The top and bottom walls form an airtight cavity which contains heat storing material, and more particularly, phase change material. The plate warmers are heated in a stacked relationship with feet on the bottom surface of the shell spacing the plate warmers apart to allow convective air flow between adjacent plate warmers. The top and bottom outer shell walls are adapted to assume substantially flat configurations to accommodate expansion of the core material when the core is heated, and to reassume their inwardly concave configurations when the core is cool. Accordingly, the outer shell members are fabricated from material sufficiently flexible to react to core expansion. More specifically, the outer shell members are fabricated from relatively thin stainless steel sheet material so that when the core is heated, it may expand by forcing the concave walls apart to assume a substantially flat configuration, and therefore, additional pressure relief means is not required. However, the heat storing core must be separately formed in a compression mold having inwardly concave molding surfaces so that the core may be molded to fit between the outer shell members which are pressed to have complementary concave surfaces. Such shaping makes fabrication and assembly somewhat complicated. In addition, the metal, i.e., stainless steel, outer shells have a relatively high heat capacity and conductivity. Consequently, the shells retain heat and are difficult to handle when heated to serving temperatures. Also, since the wax core is not encapsulated, hot wax leakage could occur along the seams of the stainless steel.
Attempts to resolve the handling problem of hot metal shells have included the use of suction cup devices or insulated gloves to prevent the user's bare hands from directly contacting the hot shell. Still other attempts have included attaching a support plate of relatively low thermally conductive materials, such as plastic, under the metal shell, thereby permitting indirect handling of the hot shell.